This invention relates to a multistage hydrodesulfurizing process for producing low sulfur distillates. A distillate boiling range feedstock containing in excess of about 3,000 wppm sulfur is hydrodesulfurized in a first hydrodesulfurizing stage containing one or more reaction zones in the presence of hydrogen and a hydrodesulfurizing catalyst. The liquid product stream thereof is passed to a first separation stage wherein a vapor phase product stream and a liquid product stream are produced. The liquid product stream, which has a substantially lower sulfur and nitrogen content then the original feedstream is passed to a second hydrodesulfurizing stage also containing one or more reaction zones where it is reacted in the presence of hydrogen and a second hydrodesulfurizing catalyst at hydrodesulfurizing conditions. The liquid product stream from this second hydrodesulfurizing stage is passed to a third reaction stage containing one or more reaction zones wherein said liquid product stream is hydrogenated in presence of a third catalyst in the presence of hydrogen and at hydrogenation conditions. The catalyst in any one or more reaction zones is a bulk multimetallic catalyst comprised of at least one Group VIII non-noble metal and at least two Group VIB metals wherein the ratio Group VIB metals to Group VIII non-noble metal is from about 10:1 to about 1:10.
Environmental and regulatory initiatives are requiring ever lower levels of both sulfur and aromatics in distillate fuels. For example, proposed sulfur limits for distillate fuels to be marketed in the European Union for the year 2005 is 50 wppm or less. There are also regulations that will require lower levels of total aromatics in hydrocarbons and, more specifically, to lower levels of multiring aromatics found in distillate fuels and heavier hydrocarbon products. Further, the maximum allowable aromatics level for U.S. proposed diesel, CARB reference diesel, and Swedish Class I diesel are 35, 10 and 5 vol. %, respectively. Further, the CARB and Swedish Class I diesel fuels allow no more than 1.4 and 0.02 vol. % polyaromatics, respectively. Consequently, much work is presently being done in the hydrotreating art because of these proposed regulations.
Hydrotreating, or in the case of sulfur removal, hydrodesulfurization, is well known in the art and typically requires treating the petroleum streams with hydrogen in the presence of a supported catalyst at hydrotreating conditions. The catalyst is usually comprised of a Group VI metal with one or more Group VIII metals as promoters on a refractory support. Hydrotreating catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally contain molybdenum or tungsten on alumina promoted with a metal such as cobalt, nickel, iron, or a combination thereof Cobalt promoted molybdenum on alumina catalysts are most widely used when the limiting specifications are hydrodesulfurization, while nickel promoted molybdenum on alumina catalysts are the most widely used for hydrodenitrogenation, partial aromatic saturation, as well as hydrodesulfurization.
Much work is also being done to develop more active catalysts and improved reaction vessel designs in order to meet the demand for more effective hydroprocessing processes. Various improved hardware configurations have been suggested. One such configuration is a countercurrent design wherein the feedstock flows downwardly through successive catalyst beds counter to upflowing treat gas, which is typically a hydrogen containing treat-gas. The downstream catalyst beds, relative to the flow of feed, can contain high performance, but otherwise more sulfur sensitive catalysts because the upflowing treat gas carries away heteroatom components, such as H2S and NH3, that are deleterious to sulfur and nitrogen sensitive catalysts.
A family of compounds related to hydrotalcites, e.g., ammonium nickel molybdate, has been prepared as an approach to improved hydrotreating catalysts. Whereas X-ray diffraction analysis has shown that hydrotalcites are composed of layered phases with positively charged sheets and exchangeable anions located in the galleries between the sheets, the related ammonium nickel molybdate phase has molybdate anions in interlayer galleries bonded to nickel oxyhydroxide sheets. See, for example, Levin, D., Soled, S. L., and Ying, J. Y., Crystal Structure of an Ammonium Nickel Molybdate prepared by Chemical Precipitation, Inorganic Chemistry, Vol. 35, No. 14, p. 4191-4197 (1996). The preparation of such materials also has been reported by Teichner and Astier, Appl. Catal. 72, 321-29 (1991); Ann. Chim. Fr. 12, 337-43 (1987), and C. R. Acad. Sci. 304 (II), #11, 563-6 (1987) and Mazzocchia, Solid State Ionics, 63-65 (1993) 731-35.
Now, when molybdenum is partially substituted for by tungsten, an amorphous phase is produced which upon decomposition and, preferably, sulfidation, provides enhanced hydrodenitrogenation (HDN) catalyst activity relative to the unsubstituted (Nixe2x80x94Mo) phase.
In accordance with this invention there is provided a multistage hydroprocessing process comprising:
a) reacting a feedstream in a first hydroprocessing stage in the presence of a hydrogen-containing treat gas, the first hydroprocessing stage containing one or more reaction zones, each reaction zone operated at first stage hydroprocessing conditions and in the presence of a hydroprocessing catalyst, thereby resulting in a first liquid product stream;
b) passing the first liquid product stream to a first separation zone where a first vapor phase product stream and a first liquid phase product stream are produced;
c) reacting the first liquid phase product stream of b) in a second hydroprocessing stage in the presence of a hydrogen-containing treat gas, the second hydroprocessing stage containing one or more second stage reaction zones operated at second stage hydroprocessing conditions wherein each or reaction zone contains a bed of hydrotreating catalyst, thereby resulting in a second liquid product stream;
d) passing the second liquid product stream of step c) to a second separation zone wherein a second vapor phase stream and a second liquid phase stream are produced;
e) reacting the second liquid phase stream from d) in a third reaction stage in the presence of a hydrogen-containing treat gas, the third hydroprocessing stage containing one or more reaction zones operated at third stage hydroprocessing conditions in the presence of a third hydrotreating catalyst, in order to form a third liquid product stream;
f) passing the third liquid product stream to a third separation zone wherein a third vapor phase stream and a third liquid phase stream are produced; and
g) collecting both the third vapor phase stream and the third liquid phase stream; and
wherein at least one of the reaction zones of at least on of said hydrodesulfurizing stages contains a bulk multimetallic catalyst comprised of at least one Group VIII non-noble metal and at least two Group VIB metals and wherein the ratio of Group VIB metal to Group VII non-noble metal is from about 10:1 to about 1:10.
In a preferred embodiment of the present invention the Group VIII non-noble metal is selected from Ni and Co and the Group VIB metals are selected from Mo and W.
In another preferred embodiment of the present invention two Group VIB metals are present as Mo and W and the ratio of Mo to W is about 9:1 to about 1:9.
In yet another preferred embodiment of the present invention the bulk multimetallic is represented by the formula:
(X)b(Mo)c(W)dOz
wherein X is one or more Group VIII non-noble metal, and the molar ratio of b:(c+d) is 0.5/1 to 3/1, preferably 0.75/1 to 1.5/1, more preferably 0.75/1 to 1.25/1.
In still another preferred embodiment of the present invention the molar ratio of c:d is preferably  greater than 0.01/1, more preferably  greater than 0.1/1, still more preferably 1/10 to 10/1, still more preferably 1/3 to 3/1, most preferably substantially equimolar amounts of Mo and W, e.g., 2/3 to 3/2; and z=[2b+6(c+d)]/2.
In another preferred embodiment of the present invention the essentially amorphous material has a unique X-ray diffraction pattern showing crystalline peaks at d=2.53 Angstroms and d=1.70 Angstroms.
In still another preferred embodiment of the present invention the Group VIII non-noble metal is nickel.
In still another preferred embodiment of the present invention at least a portion of the vapor product stream from the first separation zone is recycled to the first hydrodesulfurization stage.
In another preferred embodiment of the present invention at least a portion of the vapor product stream from the second separation stage is cascaded to said first hydrodesulfurization stage.